ORIGINAL: Campy
As for your 6" thick foam, why not just get some Home Depot, Lowes, etc. 2" x 24" x 8' pieces, cut them 6" x 4' and fasten 3 of them together with carpet tape ?
I do this when I need 4" or 6" thick pieces of foam for various projects.
Just make sure you WIPE ALL the foam dust off before applying the tape. (Blowing the dust off doesn't get it all off )
I have not had any of them come apart (actually, you need to get a thin blade knife in there to separate the pieces ).
I've considered thi,a nd have poly glued sheets before. My only concern is I don't know how well a hot wire cutter will go through the glued sections. I don't want any bumps.